1. Field of the Invention
This invention relates to a journal bearing. While the bearing is not restricted to any particular use, it is especially advantageous when used in construction equipment such as power shovels and bulldozers.
2. Description of the Related Art
Certain types of construction equipment, such as power shovels or bulldozers, are equipped with tracks on their undercarriages to enable the equipment to travel over irregular surfaces and ascend steep grades. The weight of the construction equipment is transmitted to the tracks by shafts received in journal bearings mounted on the undercarriage, e.g., track rollers, and idlers.
Journal bearings used in construction equipment need to be able withstand large loads due to the large weight of such equipment and to be able to operate for long periods without seizing. In the past, journal bearings for construction equipment have typically been made from the bearing alloy LBC-3 (Cu-10Sn-10Pb), which has copper as a main component.
A journal bearing may be manufactured entirely of a bearing alloy, but a bearing alloy generally has copper, which is expensive and has limited mechanical strength, as a main component, so manufacturing a journal bearing entirely from a bearing alloy is not appropriate from the standpoints of economy and mechanical strength, particularly when the bearing is to be used in construction equipment. Therefore, a journal bearing for use in construction equipment typically has a bimetallic structure comprising a steel backing plate and a thin layer of a bearing alloy bonded to the backing plate. Steel is particularly suitable for use as the backing plate because it is inexpensive and has excellent mechanical strength.
Methods for joining a bearing alloy to a steel backing plate include the pressure method, the melting method, and the sintering method.
In the pressure method, a bearing alloy is formed into a sheet, and the bearing alloy sheet and a steel backing plate are stacked on each other and rolled with a strong force between rollers. The rolling causes the bearing alloy and the steel backing plate to approach each other on a molecular level and be joined to each other. A bimetallic material obtained in this manner is then punched with a press to obtain a rectangular member of a prescribed size, and then the rectangular bimetallic member is formed into a cylinder to obtain a journal bearing. In this method, the bonding strength between the bearing alloy and the steel backing plate can not be made sufficiently strong, so when a journal bearing obtained by this method is used in construction equipment and a high load is applied to it, there are cases in which the bearing alloy layer peels off the steel backing plate. Accordingly, a journal bearing obtained by the pressure method is not suitable for use in construction equipment.
In the melting method, a molten bearing alloy is cast atop a steel backing plate to join the backing plate and the bearing alloy to each other. In this method, the bearing alloy and the backing plate are metallically and strongly bonded to each other. However, when the bearing alloy has cooled and solidified, it has oxides remaining on its surface, so during solidification at the time of cooling, shrinkage cavities or irregularities can form in the surface thereof, so the surface of the bearing alloy must be machined to a considerable depth to smooth it. In addition, the steel backing plate is abruptly heated by the molten bearing alloy which is at a high temperature, and its surface oxidizes or deforms due to thermal strain, so the backing plate must also be machined to smooth it. A bimetallic material which is obtained in this manner is then punched with a press to obtain a rectangular member of a prescribed size, and then the rectangular member is formed into a cylinder to obtain a journal bearing. The melting method can obtain a strong bonding strength between the bearing alloy and the steel backing plate, but it is very troublesome to perform, and at the time of casting the molten bearing alloy, oxides can be included or shrinkage cavities can form, and there were cases in which these had an adverse effect on the bearing properties.
In the sintering method, a bearing alloy in the form of a powder is dispersed on a steel backing plate and is heated without being melted to join the powder to the backing plate by a diffusion phenomenon. A bimetallic member which is obtained by the sintering method has a strong bonding strength between the bearing alloy and the steel backing plate, and the sintering temperature can be made low, so there are few thermal effects on the steel backing plate or the bearing alloy. In addition, it is the most suitable method for manufacturing a journal bearing without defects such as inclusion of oxides or shrinkage cavities. Below, the steps in manufacturing a journal bearing by the sintering method will be briefly explained.
(1) Powder Dispersing Step
A bearing alloy powder is uniformly dispersed atop a backing in the form of a steel strip.
(2) First Sintering Step
The strip on which the bearing alloy powder is dispersed is passed through a sintering furnace. The sintering furnace is at a temperature slightly higher than the solidus temperature of the bearing alloy so as not to completely melt the bearing alloy, and the interior of the furnace is filled with a reducing atmosphere of a reducing gas such as hydrogen gas. If the strip on which the bearing alloy powder is dispersed is heated under these conditions, the bearing alloy powders are joined to each other and the steel backing plate and the bearing alloy powder are joined to each other by the diffusion phenomenon.
(3) First Rolling Step
In the first sintering step, the bearing alloy powder which is dispersed on the metal plate is not completely melted, and the spaces between the bearing alloy powder form a porous bearing alloy layer, so in this state, the strip can not be used as a journal bearing. This is because the bearing alloy layer is porous, so when a high load is applied to it, the porous portion breaks down and deforms, contact between the bearing alloy layer and a shaft received by the bearing becomes non-uniform, i.e., it becomes lopsided, and the shaft may be damaged. Therefore, rolling is carried out to crush the porous portion. First rolling is carried out by passing the strip which was sintered in the first sintering step between a pair of upper and lower rollers. It is necessary to completely crush the porous portion present on the bearing alloy layer of the strip, so rolling is carried out with a large reduction ratio.
(4) Second Sintering Step
If a journal bearing is manufactured by carrying out only the first rolling, there are cases in which breakdown occurs from the interior of the bearing layer and the bearing layer peels off. The cause of this breakdown is that the porous portion which was crushed by the first rolling step is not metallically bonded to itself or to the backing, and when a high load is applied, breakdown occurs in the porous portion. Therefore, a second sintering step is carried out in order to metallically join the crushed porous portion to itself and to the backing. Second sintering is carried out under the same conditions as for the first sintering step.
(5) Second Rolling Step
During the second sintering step, the bearing alloy and the steel backing plate are annealed and their hardness decreases, so if the strip were used as a bearing with no further treatment, under a heavy load, the bearing alloy layer could be abraded or deformed. Therefore, second rolling is carried out after the second sintering step in order to increase the hardness and to adjust the thickness of the strip. The second rolling step is carried out with the same rollers as for the first rolling step, but the rolling reduction is smaller than for the first rolling step.
(6) Mechanical Working Step
A strip which is given a prescribed hardness and thickness by the second rolling step is cut to a prescribed width and length, and it is then subjected to mechanical working including rough rounding, finish rounding, machining of the interior surface, and the like to obtain a journal bearing. When the journal bearing is used in the undercarriage of tracked construction equipment, an annular flange is usually joined to one end of the journal bearing.
As mentioned above a conventional journal bearing typically employs LBC-3 (Cu-10Sn-10Pb) as a bearing alloy. LBC-3 is an appropriate material for a journal bearing used to support a shaft operating at a high speed and under a low load, but when LBC-3 is used in a journal bearing which supports a shaft operating at a low speed and under a high load such as in construction equipment, there are cases in which seizing occurs. In addition, LBC-3 contains lead, so it is potentially a source of environmental pollution. For example, when a power shovel is used for construction of a river embankment, a journal bearing made of LBC-3 may cause not only water pollution due to oil leaks but also lead contamination. If a journal bearing and a shaft in the undercarriage of construction equipment undergo seizing, the journal bearing reaches a high temperature due to the heat of friction. The heat may cause an oil seal to deteriorate, and lubricating oil may leak from the seal and mix with river water. During use of construction equipment, lead in a bearing alloy made of LBC-3 is abraded and dispersed in oil, or the lead is heated to a high temperature and is chemically modified by the oil and is dissolved in the oil. Accordingly, if construction equipment leaks oil during construction at a river embankment, oil which contains lead mixes with river water. If the river water is used as drinking water by humans, the lead can accumulate in the human body and cause lead poisoning.
Another possibility for environmental pollution occurs when equipment including bearings containing an LBC-3 bearing alloy is discarded. When construction equipment breaks down or becomes old and can no longer function adequately, it is disassembled, and parts which can still function are reused. In a device such as construction equipment in which much of the constituent components are metal, the metals are classified by type and recovered. However, in a journal bearing, the bearing alloy and the steel backing plate are metallically bonded to each other, so they cannot be separated from one another, so a journal bearing cannot be reused as either a steel plate or as a copper alloy. Journal bearings which can not be reused can only be disposed of by burial, in which case the lead becomes an environmental problem. Namely, if a lead-containing journal bearing is buried underground and comes into contact with acid rain (rain having a high acidity due to dissolving oxides of sulfur and nitrogen present in the atmosphere), the acid rain dissolves lead from the bearing alloy in the journal bearing, and the dissolved lead seeps into the ground and pollutes underground water. If underground water which is polluted by lead in this manner is drunk by humans, there is the danger of its causing lead poisoning. Therefore, in recent years, there has been a strong desire in the construction equipment industry for a journal bearing which does not contain lead, i.e., a so-called xe2x80x9clead-free journal bearingxe2x80x9d.
In LBC-3 used in conventional journal bearings, lead is scattered in the matrix of the copper alloy. As a shaft rotates in the journal bearing, the lead is stretched and becomes thin as if the surface of the bearing alloy were covered with oil, and this causes the shaft to smoothly rotate. Lead which performs this action is described as a solid lubricant. The present inventors found that even if there is not a solid lubricating effect by covering the surface of the bearing alloy with lead, if the properties of the bearing alloy are changed, the bearing alloy can withstand a heavy load without seizing taking place. Specifically, the inventors found that if a small amount of Ag is added to a Cuxe2x80x94Sn alloy, excellent bearing properties not possessed by LBC-3 are obtained.
According to one form of the present invention, a lead-free journal bearing comprises a cylindrical ferrous backing and a bearing alloy layer comprising a bearing alloy powder consisting essentially of 7-13 mass percent of Sn, 0.7-2 mass percent of Ag, and a remainder of Cu sintered to an inner surface of the backing.
According to another form of the present invention, a lead-free journal bearing comprises a cylindrical ferrous backing and a bearing alloy layer comprising a bearing alloy powder consisting essentially of 7-13 mass percent of Sn, 0.7-2 mass percent of Ag, at most 0.5 mass percent of molybdenum disulfide and/or at most 2 mass percent of graphite, and a remainder of Cu sintered to an inner surface of the backing.
In preferred embodiments, the bearing alloy layer preferably has a Vickers hardness of at least 100 HV, and the joining strength between the backing and the bearing alloy layer is preferably at least 200 MPa.
The bearing includes at least a cylindrical portion for receiving a shaft and supporting a radial load. The bearing may also include other portions, such as an annular flange disposed at one end of the cylindrical portion for receiving a thrust load.